A mobile terminal may support voice and data communications using circuit switched networks and/or SIP (Session Initiation Protocol) networks. In some instances, it may be necessary to transfer all media or a media component of the communication between the circuit switched network and the SIP network. In other instances, it may be necessary to add media components using one network technology to an existing communication using another network technology. These particular instances may be a result of the nature of the communication, the access technology being used, and the QoS (Quality of Service) requirements of the communication.
A GRUU (Globally Routed User Agent URI (Uniform Resource Identifier)) is used to uniquely identify a terminal instance involved in a communication. The GRUU is a SIP (Session Initiated Protocol) URI currently being defined within the IETF (Internet Engineering Task Force) [J. Rosenberg, Obtaining and Using Globally Routable User Agent (UA) URIs (GRUU) in the Session Initiation Protocol (SIP), Internet Engineering Task Force, Feb. 21, 2005, expires on Aug. 22, 2005](hereinafter referred to as “Rosenberg” and hereby incorporated by reference in its entirety) for the purpose of uniquely identifying a single SIP device or user agent instance where multiple SIP devices may share the same public user identity or AOR (address of record). A SIP User Agent indicates that it supports the GRUU when it registers with the SIP Registrar and also provides a unique instance identifier that uniquely identifies the SIP User Agent. The SIP Registrar then generates a GRUU based on the unique instance identifier and the registered AOR and provides it to the SIP User Agent in a “SIP 200 OK” response to the SIP register request. The GRUU can then be used as a SIP URI to communicate with just that particular SIP User Agent even though multiple SIP User Agents may be registered against the same public AOR.